Browsing by Author "Ma, Ling"

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    Auditory Streaming: Behavior, Physiology, and Modeling
    (2011) Ma, Ling; Shamma, Shihab A; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Auditory streaming is a fundamental aspect of auditory perception. It refers to the ability to parse mixed acoustic events into meaningful streams where each stream is assumed to originate from a separate source. Despite wide interest and increasing scientific investigations over the last decade, the neural mechanisms underlying streaming still remain largely unknown. A simple example of this mystery concerns the streaming of simple tone sequences, and the general assumption that separation along the tonotopic axis is sufficient for stream segregation. However, this dissertation research casts doubt on the validity of this assumption. First, behavioral measures of auditory streaming in ferrets prove that they can be used as an animal model to study auditory streaming. Second, responses from neurons in the primary auditory cortex (A1) of ferrets show that spectral components that are well-separated in frequency produce comparably segregated responses along the tonotopic axis, no matter whether presented synchronously or consecutively, despite the substantial differences in their streaming percepts when measured psychoacoustically in humans. These results argue against the notion that tonotopic separation per se is a sufficient neural correlate of stream segregation. Thirdly, comparing responses during behavior to those during the passive condition, the temporal correlations of spiking activity between neurons belonging to the same stream display an increased correlation, while responses among neurons belonging to different streams become less correlated. Rapid task-related plasticity of neural receptive fields shows a pattern that is consistent with the changes in correlation. Taken together these results indicate that temporal coherence is a plausible neural correlate of auditory streaming. Finally, inspired by the above biological findings, we propose a computational model of auditory scene analysis, which uses temporal coherence as the primary criterion for predicting stream formation. The promising results of this dissertation research significantly advance our understanding of auditory streaming and perception.
  • Thumbnail Image
    Item
    AN INFRARED AND LASER RANGE IMAGING SYSTEM FOR NON-INVASIVE ESTIMATION OF INTERNAL COOKING TEMPERATURE IN POULTRY FILLETS
    (2003-12-16) Ma, Ling; Tao, Yang; Lo , Y. Martin; Johnson, Arthur T; Wheaton, Fredrick W; Biological Resources Engineering
    Foodborne diseases caused by undercooked poultry products are noteworthy problems that have motivated research into the assessment of the endpoint temperature in meat. In this research, a novel infrared and laser range imaging system was proposed to estimate the internal cooking temperature of chicken breasts. It consisted of three subsystems: an IR imaging system, a laser range system, and an artificial neural network modeling system. Our experiments showed that geometric variables played an important role in the endpoint temperature estimation. The accuracy achieved by our system was 1.54° C for mean absolute error, 2% for mean absolute percent error, and 3.08(° C)2 for mean square error. The combined IR and laser range imaging system showed the potential for real-time, non-contact and non-invasive estimation of the internal cooking temperature in meat for enhanced food quality and safety.
  • Thumbnail Image
    Item
    Large and Stable Transmembrane Pores from Guanosine-Bile Acid Conjugates
    (Journal of the American Chemical Society, 2008-02-15) Ma, Ling; Melegari, Monica; Colombini, Marco; Davis, Jeffery
  • Thumbnail Image
    Item
    Synthetic Ion Channels From Lipophilic Guanosine Derivatives
    (2009) Ma, Ling; Davis, Jeffery T; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Synthetic ion channels and pores not only represent models of natural transmembrane ion channels, but also demonstrate their potential applications in the areas of drug delivery, biosensors, antimicrobial agents and other molecular devices. In this thesis, lipophilic guanosine derivatives that combine both "molecular recognition" and "membrane soluble" features are utilized for the development of the self-assembled synthetic ion channels. The potential of lipophilic G-quadruplexes to function as synthetic ion channels has been investigated by tracing the cation exchange process between free cations and G-quadruplex bound cations. Cation exchange between bulk cations (K+, NH4+) in solution and the bound cations in G-quadruplexes (G 1)16*4Na+*4DNP- was investigated by electrospray ionization mass spectrometry and by 1H , 15N NMR spectroscopy. The ESI-MS and 1H NMR data showed that G-quadruplexes containing "mixed cations" formed through a sequential ion exchange process. The use of NMR-"visible" 15NH4+ cations in the NMR titration experiments allowed the determination of two "mixed-cation" intermediates by 15N-filtered 1H NMR and selective NOE spectroscopy. A "central insertion" pathway was proposed for the cation exchange process from (G 1)16* 4Na+* 4DNP- to (G 1)16* 4NH4+* 4DNP-. In the lipophilic G-quadruplex, the "central" Na+, bound between the 2 symmetry related G8-Na+ octamers, is bound less strongly than are the 2 "outer" Na+ ions sandwiched within the G8-octamers. These results demonstrated the dynamic nature of lipophilic G-quadruplex in solution and directed the design of a ditopic guanosine-sterol conjugate as an approach toward making synthetic ion channels. Guanosine-sterol conjugate 3-1 was prepared by coupling 2', 3'-bis-TBDMS, 5'-amino guanosine with a bis-lithocholic acid derivative. Voltage clamp experiments demonstrated a series of stable, single ion channel conductances when compound 3-1 was incorporated into a planar phospholipid membrane. These channels are large; with nanoSiemens conductance values and they last for seconds of "open" time. This feature distinguishes them from most synthetic channels, which typically conduct in the picosiemens range with millisecond lifetimes. The structural studies using the bis-lithocholamide linker demonstrated that the guanosine moiety plays an essential role in the self-assembly of the transmembrane ion channels. The sizes of the most prevalent single channels calculated by Hille's equation are much larger than the diameter of a G-quartet, which suggested that the ion transport proceeded through larger pore(s) that form upon self-assembly of lipophilic guanosine-lithocholate 3-1 within the phospholipid membrane. The large transmembrane pore(s) could be envisioned as a supramolecular structure with hydrophobic walls of bis-lithocholate linker and a central pillar of a cation-filled G-quadruplex. The use of a bis-urea functionality in the bis-lithocholic acid linker generated guanosine-sterol conjugate 4-1. The ion channel activity of 4-1 was demonstrated by voltage clamp experiment. Large ion channels formed from 4-1 had longer life-times than those formed from compound 3-1. The extra stabilization of self-assembled ion channels attributed to the bisurea hydrogen bonding is consistent with the structural hypothesis of ion channels. The stable large transmembrane ion channels self-assembled by lipophilic guanosine derivatives have potential for delivery of drugs or biomolecules.